What marks out a great bass player FOR ME (YMMV etc etc) is someone who makes the whole band sound good. It is feel rather than technique. Technique cannot ever hurt, but it is only a tool. Many guitarists can shred like food processors, but they will only ever be guitarists and not musicians. The difference is painfully obvious when you have two guitarists in the same band, one being a guitarist and the other being a musician who plays guitar. A decent technique is not rocket science and should be achievable fairly quickly, but sympathy and empathy to the song and the band takes real-time gigging and band rehearsal. It's like playing with a good drummer - you do not notice it until a bad drummer comes in.
Why do Jazzers fill the ranks of the session pros? Partly because they have learnt to listen to what goes on around them and making it work.
Just some musings